1. Field of the Invention
This invention relates to a curcuit for operating a high intensity, gaseous-discharge lamp without requiring a large ballast transformer, and more specifically, for operating such a lamp at a frequency higher than acoustic resonance for such lamps.
2. Description of the Prior Art
Conventional ballasting of high intensity discharge lamps, such as metal-additive arc lamps, employ transformer-like coils, capacitors, or inductor coils in various combinations to provide proper voltage for starting and limiting the current during operation. Such ballasts are large, relatively expensive, and not efficient at low cost. Simple inductor ballasts are available; however, they provide poor regulation for line voltage variations.
Regulating solid state ballasts have been developed, but heretofore no commercial ballasts have been developed which is suitable for the operating conditions of high pressure mercury, sodium and metal halide lamps to give proper control of lamp wattage for high ranges of lamp voltages, line fluctuations and temperatures.
Although theoretically a lamp may be operated on a combination of applied dc and ac, which would give lower noise than ac alone, it has been discovered that the application of dc is bad for lamp efficiency and life. The application of low audio frequency ac causes noisy ballast conditions. The application of medium frequency ac causes noisy and unstable lamp conditions. In fact, the high pitch whine of lamps operated under such conditions is extremely unpleasant. Therefore, it has not been recognized that high frequency ac, i.e. frequencies above acoustic resonance, may be used with regard to lamps; however, life tests and lumen tests have revealed that high frequency operation beyond a certain range is perfectly satisfactory, both as to providing acceptable lamp operating stability and an absence of audible noise.
As used hereinafter in this application, the term "frequencies above acoustic resonance" is understood to mean frequencies above which the phenomenon of acoustic resonance no longer produces visual disturbances to the emission of light from the gaseous discharge lamp; i.e., above the highest band of acoustic resonant frequencies which either extinguish the lamp or cause unpleasant flickering of the light.
Therefore, it is a feature of this invention to provide an improved operating circuit for a high intensity gaseous discharge lamp that provides a high frequency mode of operation, such circuit employing a high-Q coil to provide the requisite high power at high frequency required for operation.
It is another feature of the present invention to further provide such a circuit that would not injure the lamp or a circuit component either under open or shorted lamp conditions.
It is yet another feature of this invention to provide a circuit that operates under 10-to-1 lamp impedance (wide range of load conditions) change during warm-up and having microsecond lamp ignition speed.